Highly precise and sophisticated polishing of substrate surfaces is necessary in many cases including for substrates such as glasses for flat panel display, magnetic disks and silicon wafers for semiconductor, and insulating films, metal layers, barrier layers and the like which are formed in semiconductor device manufacturing steps. Such polishing is usually carried out using a polishing liquid comprising abrasive grains including silica, alumina, zirconia or cerium oxide, for example, according to the purpose of use.
The progressively high integration and micronization in semiconductor device manufacturing steps in recent years is further reducing the permissible sizes of scratches generated on the surface to be polished during polishing. It is therefore becoming more difficult to reduce scratches.
A variety of different abrasive grains are used to reduce scratches. For example, polishing liquids widely used for polishing of inorganic insulating films such as silicon oxide films in semiconductor device manufacturing steps are polishing liquids comprising cerium oxide as abrasive grains (see Patent document 1, for example). Cerium oxide has a lower hardness than silica or alumina, and produces fewer scratches on surfaces to be polished, and it is therefore widely used as abrasive grains.
It is also often attempted to reduce the abrasive grain particle sizes in order to reduce scratches. However, reducing the abrasive grain particle sizes lowers the mechanical effect of the abrasive grains, and thus reduces the polishing rate. Therefore, it is attempted, with abrasive grains used in the prior art, to achieve both increased polishing rate and reduced scratches by controlling the particle sizes of the abrasive grains, but it is still extremely difficult to reduce scratches. This problem is being studied with the aim of achieving both increased polishing rate and reduced scratches by using a polishing liquid comprising tetravalent cerium hydroxide particles (see Patent document 2, for example), and research is also being conducted on methods for producing tetravalent cerium hydroxide particles (see Patent document 3, for example).
In order to achieve flat polishing of inorganic insulating films with irregularities, an excellent polishing selective ratio of the film to be polished, such as a silicon oxide film, with respect to the stopper film, such as a silicon nitride film (polishing rate ratio: polishing rate for film to be polished/polishing rate for stopper film) is required for polishing liquids comprising such tetravalent cerium hydroxide particles. Various additives has been added to polishing liquids and investigated in order to solve this problem. For example, polishing liquids that can improve the flatness of polished surfaces have been proposed, wherein anionic additives are added to the polishing liquids for a satisfactory polishing rate for silicon oxide films with essentially no polishing of silicon nitride films (namely, having polishing selectivity of silicon oxide films to silicon nitride films) (see Patent document 4, for example).
A variety of additives have thus been investigated in order to adjust the properties of polishing liquids, but depending on the additives used, there has sometimes been a trade-off between the effect obtained by the additives and reduction in polishing rate.